Normally, the life cycle involves dogs. People are infected only accidentally.
1. Infected dogs excrete Toxocara eggs in their stool.
2. After being excreted into the environment, the eggs mature and become able to cause infection.
3. A dog swallows eggs, which then hatch and release larvae.
4. The larvae penetrate the wall of the intestine. In younger dogs, the larvae travel through the lungs to the windpipe. They are coughed up, swallowed, and returned to the small intestine, where they mature into adult worms. There, adult female worms produce eggs. In older dogs, the larvae tend to form cysts in tissues.
5. If an infected dog becomes pregnant, larvae in cysts may be reactivated late in the pregnancy. The larvae can infect the puppies by crossing the placenta before birth or by entering the mother's milk to be consumed by nursing puppies.
6. Puppies are a common source of eggs in the environment.
7. Toxocara eggs may be ingested by other mammals, such as rabbits. In these mammals, the eggs hatch into larvae, which penetrate the wall of the intestine and travel to various tissues where they form cysts.
8. The life cycle is completed when dogs eat these mammals, and the larvae develop into egg-laying adult worms in the dog's small intestine.
9–10. People can be accidentally infected if they swallow eggs in contaminated soil or in undercooked meat from infected animals. After people swallow the eggs, the eggs hatch into larvae.
11. The larvae penetrate the wall of the intestine and travel to various tissues (such as the liver, heart, lungs, brain, muscle, or eyes). In people, the larvae do not develop into adult worms. However, they can damage tissues and cause inflammation.
Image from the Centers for Disease Control and Prevention, Global Health, Division of Parasitic Diseases and Malaria.